The big picture: Plants evolved from green algae


  • Cyanobacteria & protist made landfall ~1.2 bya
    • plants, fungi and animals ~500 mya
    • first forests 385 mya


  • Plants evolved from green algae
    • Several key ‘derived shared traits’


  • Plants support all life on earth
    • Oxygen to breath
    • food to eat
    • new habitats

Green Algae



  • Sister group to land plants
    • marine and freshwater
    • single and multi-cellular
    • broad, thick filaments
    • some have AoG
    • evolved around ~750mya


  • Reproductive features:
    • Oogonia = egg containing cell
    • Antheridium = sperm containing cell

Molecular evidence points at charophytes (freshwater)

Land plants are a monophyletic group

All land plants are embryophytes




  • 10 divisions of land plants
    • does not include algae
    • divisions are species diverse


  • Zygote develops into multi-cell embryo
    • enclosed in reproductive structure
    • algae do not retain embryo


  • Land plant & embryophyte terms used interchangeably

Shared traits define land plant evolution

Multi-cellular, Dependent embryos (placental transfer)

Waxy cuticle and stomata

Multi-cellular Gametangia

Photosynthesis with unique pigments (chlorophyll A & B)

Unique cell walls




  • Cells walls made of cellulose
    • not unique to plants


  • Pectin to fortify cell walls
    • unique to plants


  • Produce cells walls in unique way
    • at end of mitosis

Alternation of generations


  • Land plants alternate between diplod (2N) and haploid (1N) generations
    • diploid = sporophyte
    • haploid = gametophyte


  • sex cells (1N) make zygote (2N)
  • zygote → embryo → sporophyte (all mitosis)
  • sporophyte makes spores (1N) by meosis
  • spores germinate into gametophyte (1N) by mitosis
  • gametophyte makes sex cells (sperm and egg)


  • Dominance of different generations key for evolution

First plant group: non-vascular bryophytes (mosses)


  • Mosses, hornworts and liverworts
    • Fossils of bryophyte spores ~470mya


  • Non-vascular; ground hugging carpets
    • bodies to thin to support height growth


  • Have a rhizoid but not a root
    • anchors plant
    • does not uptake water


  • Need H20 for swimming sperm

Bryophytes have gametophyte dominated life cycle

Vascular plants were the next evolutionary step

  • Vascular plants show up ~425mya
    • gametophytes become reduced


  • Developed a vascular system
    • grow taller = get light
    • xylem and phlome conduct


  • Developed leaves & roots
    • better access to resources


  • First vascular plants lacked seeds
    • sperm still needed to swim
    • ferns and lycophytes

Lycophytes: First vascular plants



  • Independent, branched sporophyte
    • now with roots


  • Lignified vascular tissue
    • xylem and phloem


  • Leaves are microphylls
    • one vein per leaf (unique trait)


  • Lycophyte forests (30m) once existed
    • gives us coal!

Ferns


  • Whisk Ferns, Equisetum & Ferns


  • Leaves are megaphylls
    • many veins per leaf


  • Best plants ever!


  • New Gaga genus with 19 species
    • “We wanted to name this genus for Lady Gaga because of her fervent defense of equality and individual expression” Pryer @ Duke University

Seedless vascular plants have free-living separate generations



  • Sporophyte: The fern you see in the woods
    • becomes dominant stage


  • Gametophye: independent haploid generation
    • start out female or bisexual


  • Derived Trait - remember how a moss looks!


  • What does natural selection favor diploid dominant life cycles?

Seed plants were the next evolutionary step


  • Seeds changed the course of plant evolution
    • Seed plants show up ~360 mya
    • climate became drier


  • Seeds consists of embryo & nutrients
    • surrounded by protective coat
    • seeds can be dispersed long distances
    • less dependence on water


  • 2 living seed lineages
    • gymnosperms (naked seed)
    • angiosperms (covered seed = fruits)

Why seeds matter so much…


  • Before seeds, spore was only protective structure
    • super small so spread all over Earth


  • Seeds are multicellular
    • what can this add?


  • Seeds can remain dormant for a long time


  • Gymnosperms: seeds exposed on sporophylls, form cones
  • Angiosperms: seeds enclosed inside fruits

4 living lineages of gymnosperms


  • Cycads: 300 living species
    • earliest evidence of insect pollination
    • Cycads and dinosaurs lived together


  • Gnteophytes: small and super weird group
    • some flowering traits


  • Ginkos: Ginkgo biloba is only species left!


  • Conifers: ~600 species (cones)
    • evergreen and deciduous
    • leaves are needles or scales

Flowering plant diversity


  • Most diverse living group of plants
    • 14,000 genera, 250,000 species
  • 2 classes: Moncots and Dicots
    • monocots = one seed leaf in embryo
    • dicots = two seed leaf in embryo

Angiosperm evolution: Bugs and animals